Vascular smooth muscle plays a pivotal role in the physiology and pathology of the blood vessel. Alterations in the contractile properties and phenotype of the smooth muscle cell have been implicated in such common vascular disease processes as atherosclerosis and hypertension. Despite its central role in vascular pathophysiology, very little is known about the contractile apparatus and in particular about Myosin, the principal force-producing protein. Our recent studies have focused on identifying two distinct smooth myosin heavy chain isoforms that are the products of alternate RNA splicing of a single gene. These two isoforms are co-expressed in a number of smooth muscle tissues. In addition, we have new data to indicate that a novel myosin isoform is expressed in certain smooth muscle tissues. This proposal will continue to elucidate the molecular structure, heterogeneity and the factors regulating myosin expression. A major focus of this proposal will be to understand the role of growth factor(s) (PDGF) in regulating myosin gene expression. The primary objectives of this proposal are to: I. Determine whether alternate RNA splicing events occur at the 5' end of the SMHC gene encoding SM1 and SM2 MHC isoforms. II. Characterize the new SMHC isoform at the genetic and biochemical level. III.Complete the characterization of the SMHC gene promoter by DNA sequencing and gene transfer analysis. IV. Study the effect of PDGF and serum on MHC gene expression in aortic smooth muscle cultures. V. Identify upstream promoter elements which interact with trans-acting regulatory factors to direct tissue specific transcription of the smooth muscle MHC gene. VI. Isolate master regulatory genes involved in determining smooth muscle phenotype. We anticipate that these studies should provide direct insight into the mechanisms governing vascular function and modulation at the subcellular level.